Quote:
Originally Posted by Verdoro 68
The 2bbl (7043062) on our '74 Ventura 350 has an off idle stumble that I've been trying to resolve. It's really pronounced when the engine is cold, so much that it will backfire or stall sometimes, but it's always there even when at operating temp. I figured it was going lean, so I welded in an O2 bung this afternoon and used my wideband to see what was going on.
The mixture is spiking to 15.5-17ish when I step on the pedal from idle (~14.0 AFR).
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14:1 AFR is
kinda lean for a carbureted idle. Connect a vacuum gauge to manifold vacuum, set the idle mixtures to lean best vacuum--achieve best vacuum. Best vacuum can be had with a range of AFR, you want the leanest AFR that allows best vacuum. Then lean-drop the vacuum about 1/4 inch on each of the two screws--about 1/2 inch, total. So a very slight lean-drop from lean best vacuum. A slight lean-drop eliminates a fair amount of tailpipe emissions without really affecting idle quality.
Spiking even higher at tip-in really sounds like an accelerator pump issue--but I am biased. I had perpetual problems with the accelerator pump on Mom's '77 Nova 305 with 2GV. I eventually installed a Cliff Ruggles-supplied accelerator pump which seems to have cured that problem.
Quote:
Originally Posted by Verdoro 68
I've gone through several accelerator pumps trying to diagnose this. I can see that it's getting a good squirt of fuel immediately so I don't think that's the problem.
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If the pump were truly working, tip-in should show a momentary rich condition. We used to watch for a momentary spike in HC emissions when a tailpipe-sniffer (Exhaust-gas analyzer) was hooked-up.
Quote:
Originally Posted by Verdoro 68
I'm wondering if I need bigger jets. It's got 54s in it now which I believe are from the factory.
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The carb would have been set up for E-0; and I bet you're running E10. So going slightly richer may be warranted...but I don't think main jets are the problem here.
ANY misfire for ANY reason will cause an O2 sensor to report a false-lean condition.
Plugged passage on the power valve? Failed vacuum advance; advance connected to ported vacuum? Weak ignition coil? Failed pickup coil wires on an HEI distributor?